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OCN 401

Global carbon cycle I

Systematics and history of the C cycle

The C cycle is dominated by the processes of:Silicate rock weatheringOrganic C production

CO2 is the transfer medium between these reservoirs

The time scales of the processes are: Sub-annual to millenia for organic C production

Thousands to millions of years for the rock cycle

The major reservoirs of C are: Carbonate rocks/sediments

Organic carbonDissolved inorganic C in seawater

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Global carbon cycle involves biomass on land and in the oceans,the atmosphere, the rock cycle and the physical chemistry of the oceans

Is also linked to the global cycles of oxygen, nitrogen and phosphorous

Atmospheric CO2 levels are controlled by the relative rates of these transfer processes

The atmosphere is a holding tank for CO2

The oceans contain the largest reservoirs of C

We will look at the reservoir and transfer processes to see how they regulate climate and the feedback loops that this produces

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Carbon reservoir sizes at the Earth’s surface

Carbon dioxide is only a small fraction of the reservoir, but its role in photosynthesis, climate regulation androck weathering make it a critical component of the system

The current global C cycle, reservoirs and fluxes in units of 1015g C (and yr-1)

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Listed in order of size: g CCarbonate sediments 6.53 x 1022 Organic matter in seds 1.25x 1022

DIC in Ocean 3.74 x 1019

DOC in oceans 1.00 x 1018

Ocean biota 3.00 x 1015

Carbonate sediments in the oceans are the largest reservoirlarger than organic matter reservoir by ~ 4:1

Ocean water is next largest reservoir Inorganic (DIC) is ~40 x organic ocean reservoir

Oceanic reservoirs

Listed in order of size :CaCO3 in soils 7.2 x 1017

Land biota 7.0 x 1017

Soil organic matter 2.5 x 1017

Atmosphere CO2 6 x 1017

Soils are next largest reservoirLiving biotic reservoir is ~same as inorganic reservoir

Dead organic matter is 1/3 of the inorganic reservoirPhytomass ~100 x bacteria and animal reservoirsAtmosphere is the smallest reservoir, similar to size of all living biomass

Land and atmospheric reservoirs

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The organic C cycle

Transfers between organic reservoirs (on land and in the oceans)can occur on short time scalesBuried reservoirs of organic carbon are large relative to atmosphere

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Photosynthesis and respiration

CO2 + 2H2O = CH2O + H2O + O2

Photosynthesis proceeds to the right, releases oxygen to the atmosphereRespiration proceeds to the left, consumes oxygen from the atmosphere Both of these reactions proceed rapidly on annual cycles

Annual cycle of plant growth and death moves CO2 between atmosphere and biosphere and back again

May 2016 average = 407.7 ppmv

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Reactions linking carbon and oxygen

C + O2 = CO2

Under conditions at Earth’s surface this reaction proceeds to the right,Thermodynamics favours CO2 C and O2 --lots of Gibbs free energyBut kinetics are slow, activation energy is needed to promote the reactionThe oxidation of carbon is what is happening during the burning of fossil fuels or forest fires

Organic C burial links CO2 to atmospheric O2 cycle

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The rock cycle

Carbonate and silicate rock cycle

Weathering on land

CaCO3 + CO2 + H2O = Ca2+ + 2HCO3-

CaSiO3 +2CO2 +3H2O = Ca2+ + 2HCO3- + H4SIO4

Uptake of atmospheric CO2 during weathering on land, delivery of dissolved form to oceans

Deposition in the oceans

Ca2+ + 2HCO3- = CaCO3 + CO2 + H2O

H4SiO4 = SiO2 + 2H2ORelease of CO2 during carbonate precipitation

Metamorphic reactions

CaCO3 + SiO2 = CaSiO3 + CO2Release of CO2 return to atmosphere via volcanic/hydrothermal activity

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CaCO3 weathering on land and reprecipitation in ocean has no net effect on atmospheric CO2

Weathering of silicates on land and reprecipitation in ocean results in net uptake of atmospheric CO2

Balance of weathering types affects atmospheric CO2

Subduction of sediments and volcanic activity returns CO2 to atmosphere

If no recycling, weathering would remove all CO2 from atmosphere in ~ 1 million years

Residence time of CO2 in atmosphere relative to weathering and volcanic input is ~ 6,000 years, i.e. is a long term control

Does not control decadal to century scale changes seen in modern C cycle

Geological history of the C cycle

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Long term changes in atmospheric CO2 driven by rock cycle and biological cycle

Initial high levels of atmospheric CO2--“Weak” sun

Silicate weathering and carbonate precipitation in ocean reduced atmospheric CO2 levels

Evolution of life ~3.9 Ga sequestered organic C

Leads to production of oxygen

Initial production of O2 “used up” by oxidation of Fe in seawaterTerrestrial weathering also consumed early O2 productionMulti-cellular organisms appear as O2 levels in atmosphere increase

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Phanerozoic C cycleIncrease in tectonic activity increases sea level

PannotiaBreak-up

600 mya

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500 mya

400 mya

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300 mya

Phanerozoic C cycleSea level increase leads to evolution of land plants

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C storage 400-500 Ma 30 x 1012 moles C buried in sediments yr-1

releases oxygen to atmosphere

Pannotiabreak-up

Land plants

O2 levels in atmosphere track maximum burial of organic C ~ 350 MaMicrobial processes then pull down O2 levels as oxidise organic C

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Pangaea forms then breaks up leading to an increase in tectonic activity

Pannotiabreak-up

Pangaeaforms

Land plants

Pangaeabreak up

Angiospermsevolve

Himalayasform

Evolution of angiosperms ~ 150 Ma Deeper roots increase Si weathering rates, leads to drop in atmos CO2

Pannotiabreak-up

Pangaeaforms

Land plants

Pangaeabreak up

Angiospermsevolve

Himalayasform

Ordivicianextinction

Permian extinction

Devonianextinction

Triassic extinction

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Phanerozoic C cycleDriven by evolution of land plants - C storage 400-500 Ma30 x 1012 moles C buried in sediments yr-1 releases oxygen to atmosphereAtmos. reservoir O2 38 x 1018 moles, Tr = 1 x 106 yrs wrt sedimentary CUplift of rocks and weathering of kerogen balances processAtmospheric O2 is balance of organic C burial and its weathering

PannotiaBreak-up

Land plants

Pangaeaforms

Pangaeabreak up

Angiospermsevolve

Himalayasform